1. Field of the Invention
The present invention relates to a sol of modified stannic oxidexe2x80x94zirconium oxide colloidal particles having a particle diameter of 4.5-60 nm which is formed by coating stannic oxidexe2x80x94zirconium oxide complex colloidal particles at their surfaces with colloidal particles of a tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex having a particle diameter of 2-7 nm, and also to a preparation method thereof.
The sol according to the present invention is used as a component for a hard coating agent applied on a plastic lens surface and also for other various applications.
2. Description of the Related Art
A variety of metal oxide sols are already known.
There have been used sols of metal oxides having a high refractive index as a component of a hard coating agent applied on a plastic lens surface which is recently used much in order to improve the surface.
For example, a hard coating agent containing particles of metal oxides such as Al, Ti, Zr, Sn and Sb having a size of 1-300 nm is described in Japanese Patent Publication No. Sho 63-37142.
Although a stable sol of tungsten oxide alone is not known yet, Japanese Patent Application Laid-Open No. Sho 54-52686 proposes a sol obtained by addition of silicate in which a WO3:SiO2:M2O molar ratio (wherein, M represents alkali metal atom or ammonium group) is 4-15:2-5:1.
A silicatexe2x80x94stannate complex sol in which a molar ratio of Si:Sn is 2-1000:1 has been proposed in Japanese Patent Publication No. Sho 50-40119.
Further, Japanese Patent Application Laid-Open No. Hei 3-217230 proposes a stable sol comprising a modified metal oxide colloid having a particle diameter of 4.5-60 nm and which is formed by coating colloidal particulate surfaces of metal oxide having a particle diameter of 4-50 nm and a valence of 3, 4 or 5 as nuclei with colloidal particles of a tungsten oxidexe2x80x94stannic oxide complex having a WO3/SnO2 weight ratio of 0.5-100 and a particle diameter of 2-7 nm, the stable sol containing 2-50% by weight of total metal oxides.
Further, Japanese Patent Application Laid-Open No. Hei 6-24746 proposes a stable sol of a modified SnO2-ZrO2 complex which comprises particles formed by coating colloidal particle surfaces of metal oxide having a ZrO2/SnO2 weight ratio of 0.02-1.0 and a particle diameter of 4-50 nm as nuclei with colloidal particles of a WO3-SnO2 complex having a WO3/SnO2 weight ratio of 0.5-100 and a particle diameter of 2-7 nm.
However, if these conventional metal oxide sols, particularly cationic metal oxide sols, are used as a component of a hard coating agent, the stability of the obtained hard coating agent is insufficient and also transparency, adhesion, weather resistance, etc. of a hardened coating coated with the hard coating agent are insufficient. Further, if an Sb2O5 sol is used as a component of a hard coating agent, the Sb2O5 sol can no longer improve a refractive index of the hardened coating sufficiently when a refractive index of a plastic substrate in a lens is 1.6 or more, since a refractive index of Sb2O5 is about 1.65-1.70.
A sol of tungsten oxide described in the above-mentioned Japanese Patent Application Laid-Open No. Sho 54-52686 is obtained by adding a silicate to an aqueous tungstate solution which is obtained by decationizing an aqueous tungstate solution, but it is stable only at a strong acidic state and an effect for improving a refractive index of a coating is low in the case of use as a component of a hard coating agent.
A silicatexe2x80x94stannate complex sol described in the above-mentioned Japanese Patent Publication No. Sho 50-40119 is obtained by decationizing a mixed aqueous solution of an alkali silicate and an alkali stannate, but an effect for improving a refractive index of a coating is also low in the case of use as a component of a hard coating agent as described above.
A modified metal oxide sol described in the above-mentioned Japanese Patent Application Laid-Open No. Hei 3-217230 having a refractive index of 1.7 or more is stable, thus it can be used as a component of a hard coating agent for a plastic lens and it can satisfy almost all properties required for a hard coating such as damage resistance, transparency, adhesion, water resistance, weather resistance, etc.
A modified stannic oxidexe2x80x94zirconium oxide sol described in the above-mentioned Japanese Patent Application Laid-Open No. Hei 6-24746 having a refractive index of 1.7 or more is stable, thus it can be used as a component of a hard coating agent for a plastic lens, and it can satisfy almost all properties required for a hard coating such as damage resistance, transparency, adhesion, etc.
A modified metal oxide sol according to the present invention provides a stable sol of modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles having good water resistance, moisture resistance, weather resistance and a high refractive index which overcomes yellowing due to ultraviolet radiation found when the conventional metal oxide sols are used and also overcomes such problems as water resistance and moisture resistance, and thus it provides a metal oxide sol which can be used by mixing with a hard coating paint as a component for improving properties of the hard coating applied on a plastic lens surface.
The present invention provides a stable sol which comprises modified colloidal particles of stannic oxidexe2x80x94zirconium oxide complex having a particle diameter of 4.5-60 nm, wherein the particles are formed by coating complex colloidal particles of a stannic oxidexe2x80x94zirconium oxide having a bonded structure of stannic oxide colloidal particles and zirconium oxide colloidal particles at a ZrO2/SnO2 ratio of 0.02-1.0 based on weights of these oxides and having a particle diameter of 4-50 nm as nuclei, with colloidal particles of a tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex having such a constitution that a WO3/SnO2 weight ratio is 0.1-100, that a SiO2/SnO2 weight ratio is 0.1-100 and having a particle diameter of 2-7 nm, and which contains 2-50% by weight of total metal oxides.
Further, a method for preparing a stable sol according to the present invention comprises the steps of:
a step (a) of mixing an aqueous stannic oxide sol which contains colloidal particles of stannic oxide having a particle diameter of 4-50 nm at a concentration of 0.5-50% by weight as its oxide, SnO2, with an aqueous oxyzirconium salt solution having a concentration of 0.5-50% by weight calculated as ZrO2, at a ZrO2/SnO2weight ratio of 0.02-1.0;
a step (b) of forming an aqueous stannic oxidexe2x80x94zirconium oxide complex sol having a particle diameter of 4-50 nm by heating the mixed solution obtained in the step (a) at 60-200xc2x0 C. for 0.1-50 hours;
a step (c) of forming a sol of a tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex by preparing an aqueous solution containing tungstate, stannate and silicate at a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100 and removing cations present in the solution;
a step (d) of mixing 100 parts by weight, calculated as the total of ZrO2 and SnO2 contained, of the aqueous stannic oxidexe2x80x94zirconium oxide complex sol obtained in the step (b) with 2-100 parts by weight, calculated as total of WO3, SnO2 and SiO2 contained, of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol obtained in the step (c) having a particle diameter of 2-7 nm, a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100 at 0-100xc2x0 C.; and
a step (e) of removing anions present in the aqueous modified stannic oxidexe2x80x94zirconium oxide complex sol obtained in the step (d) by bringing the sol into contact with an anion exchanger.
The sol of stannic oxidexe2x80x94zirconium oxide complex colloidal particles to be used as nuclear particles in the preparation of the sol according to the present invention can be prepared by the method comprising the above-mentioned steps (a) and (b). Stannic oxide colloidal particles to be used in the step (a) can be made easily in a sol form of colloidal particles having a particle diameter of about 4-50 nm by known methods such as an ion exchange method, a peptization method, a hydrolysis method and a reaction method.
As examples of the above-mentioned ion exchange method, there may be mentioned a method of treating stannate such as sodium stannate with a hydrogen type cation exchange resin, and a method of treating stannate such as the above-mentioned stannic chloride and stannic nitrate with a hydroxide type anion exchange resin. As examples of the above-mentioned peptization method, there may be mentioned a method of neutralizing a stannate with a base, or a method of washing a stannic hydroxide gel obtained by neutralizing stannate with hydrochloric acid, and thereafter peptizing it with an acid or a base. As examples of the above-mentioned hydrolysis method, there may be mentioned a method of hydrolyzing tin alkoxide or of hydrolyzing basic stannic chloride, a basic salt, by heating and thereafter removing an unnecessary acid. As an example of the above-mentioned reaction method, there may be mentioned a method of reacting metallic tin powders with an acid.
A medium for the stannic oxide sol may be water or any hydrophilic organic solvent, but an aqueous sol is preferable in which a solvent is water. Further, pH of the sol may be such a value to make the sol stable, generally about 0.2-11. So long as an object of the present invention is attained, optional components such as alkaline substances, acidic substances and oxycarboxylic acids may be introduced in the stannic oxide sol for stabilization of the sol. A concentration of the stannic oxide sol used may be about 0.5-50% by weight of stannic oxide, but the concentration is preferably lower, more preferably 1-30% by weight.
The stannic oxidexe2x80x94zirconium oxide complex sol can be obtained by the step (a) in which the above-mentioned stannic oxide sol is mixed with an oxyzirconium salt at a ZrO2/SnO2 weight ratio of 0.02-1.0 at 0-100xc2x0 C. for 0.5-3 hours and the step (b) in which it is heated at 60-200xc2x0 C. for 0.1-50 hours.
As the oxyzirconium salts to be used, there may be mentioned zirconium oxychloride, zirconium oxynitrate, zirconium oxysulfate, oxyzirconium salts of organic acids such as zirconium oxyacetate, and zirconium oxycarbonate. These oxyzirconium salts may be used as solids or aqueous solutions, and they may be used as aqueous solutions containing 0.5-50% by weight, preferably 0.5-30% by weight, of ZrO2. Water-insoluble salts such as zirconium oxycarbonate can be used such a case that stannic oxide to be mixed is an acidic sol.
It is preferable to use the stannic oxide sol as an alkaline sol stabilized particularly with an organic base such as amine, and the mixing with oxyzirconium salt is carried out preferably at 0-100xc2x0 C., preferably from room temperature to 60xc2x0 C. And, the mixing may be carried out by adding the oxyzirconium salt to the stannic oxide sol with stirring or by adding the stannic oxide sol to the aqueous solution of the oxyzirconium salt with stirring, wherein the latter is more preferable. It is necessary to carry out the mixing sufficiently, and the mixing period is preferably 0.5-3 hours.
WO3, SnO2 and SiO2 complex colloidal particles contained in the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol obtained in the step (c) which is used as a coating sol according to the present invention can be observed as to a particle diameter by means of an electron microscope, and the particle diameter is 1-50 nm, preferably 2-7 nm, more preferably 2-5 nm. As a dispersing medium for the sol colloidal particles, water or any hydrophilic organic solvent may be used. The sol contains WO3, SnO2 and SiO2 at a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100. A total concentration of WO3, SnO2 and SiO2 contained in the sol is generally 40% by weight or lower, in practice preferably 2% by weight or greater, preferably 5-30% by weight. The sol exhibits pH of 1-9 and it is a colorless and transparent or slightly colloidal colored solution. And, it is stable for more than 3 months at room temperature, and more than 1 month even at 60xc2x0 C. without any precipitation in the sol, and further a viscosity of the sol is not increased nor gelation is occurred.
A method for preparing a stable tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol characterized in that complex colloidal particles of tungsten oxide (WO3), stannic oxide (SnO2) and silicon dioxide (SiO2) obtained in the step (c) are contained comprises the steps of:
(c-1) preparing an aqueous solution containing tungstate, stannate and silicate at a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100; and
(c-2) removing cations present in the aqueous solution obtained in the step (c-1).
As examples of tungstates, stannates and silicates to be used in the step (c-1), there may be mentioned tungstates, stannates and silicates of alkali metals, ammonium, amines, etc. As preferable examples of these alkali metals, ammonium and amines, there may be mentioned those of Li, Na, K, Rb, Cs, NH4, alkyl amines such as ethylamine, triethylamine, isopropylamine, n-propylamine, isobutylamine, diisobutylamine and di (2-ethylhexyl) amine; aralkyl amines such as benzylamine; alicyclic amines such as piperidine; and alkanol amines such as monoethanolamine and triethanolamine. Particularly, sodium tungstate (Na2WO4.2H2O), sodium stannate (Na2SnO3.3H2O) and sodium silicate (water glass) are preferable. Further, those obtained after dissolving tungsten oxide, tungstic acid, stannic acid, silicate, etc. into an aqueous solution of alkali metal hydroxide may be used. Further, amine silicates and quaternary ammonium silicates obtained by adding alkyl amines such as ethylamine, triethylamine, isopropylamine, n-propylamine, isobutylamine, diisobutylamine and di(2-ethylhexyl)amine to active silicic acid as silicate may be used.
As the methods for preparing the aqueous solution in the step (c-1), there may be mentioned a method of preparing the aqueous solution by dissolving respective powders of tungstate, stannate or silicate into water, a method of preparing the aqueous solution by mixing an aqueous tungstate solution, an aqueous stannate solution and aqueous silicate solution, and a method of preparing the aqueous solution by adding powders of tungstate and stannate as well as an aqueous solution of silicate into water.
The aqueous solution of tungstate to be used for the sol preparation in the step (c) has preferably a WO3 concentration of about 0.1-15% by weight, but higher concentrations may be used.
The aqueous solution of stannate to be used in the preparation of the sol in the step (c) has preferably a SnO2 concentration of about 0.1-30% by weight, but higher concentrations may be used.
The aqueous solution of silicate to be used in the preparation of the sol according to the present invention has preferably a SiO2 concentration of about 0.1-30% by weight, but higher concentrations may be used.
The preparation of the aqueous solution in the step (c-1) may be carried out at from room temperature to 100xc2x0 C., preferably from room temperature to 60xc2x0 C. The aqueous solution to be mixed has preferably a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100.
The step (c-2) is a step to remove cations present in the aqueous solution obtained in the step (c-1). The decationizing treatment may be carried out by bringing it into contact with a hydrogen type ion exchanger or salting-out. The hydrogen type cation exchangers to be used herein may be any ones generally used, and it is convenient to use commercially available hydrogen type cation exchangers.
If the aqueous sol obtained via the steps (c-1) and (c-2) has a low concentration, the aqueous sol maybe concentrated, if desired, by general concentration methods such as an evaporating method and an ultrafiltration method. In particular, the ultrafiltration method is preferable. During concentrating, the temperature of the sol is preferably maintained at about 100xc2x0 C. or lower, particularly at about 60xc2x0 C. or lower.
A hydrophilic organic solvent sol referred to an organosol can be obtained by substituting water in the aqueous sol in the step (c) with a hydrophilic organic solvent.
The sol of tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex obtained in the step (c) contains complex particles comprising the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex obtained by complexing (subjecting to solid solution) tungsten oxide, stannic oxide and silicon dioxide homogenously at atomic levels. Therefore, it is not obtained only by mixing three sols, that is, a tungsten oxide sol, a stannic oxide sol and a silicon dioxide sol.
Since particles of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol form a solid solution, the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol cannot be decomposed into tungsten oxide particles, stannic oxide particles and silicon dioxide particles by solvent substitution.
The tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol has improved water resistance, moisture resistance and weather resistance compared with the tungsten oxidexe2x80x94stannic oxide complex sol when formed into a coating by coating a substrate therewith.
If the WO3/SnO2 weight ratio of the sol obtained in the step (c) is less than 0.1, the sol becomes unstable, and if the weight ratio is above 100, the sol does not exhibit stability. The oxycarboxylic acid to be added during the preparation of the above-mentioned organosol from the aqueous sol having high pH may contribute to stabilization of the sol, but water resistance of a dried coating obtained by using the sol becomes low if an addition amount thereof is 30% by weight or higher based on the total of WO3, SnO2 and SiO2. As examples of oxycarboxylic acids to be used, there may be mentioned lactic acid, tartaric acid, citric acid, gluconic acid, malic acid, glycolic acid, etc. Further, as alkali components, there may be mentioned, hydroxides of alkali metals such as Li, Na, K, Rb and Cs, NH4, alkyl amines such as ethylamine, triethylamine, isopropylamine and n-propylamine; aralkyl amines such as benzylamine; alicyclic amines such as piperidine; and alkanol amines such as monoethanolamine and triethanolamine. These may be contained by mixing two or more thereof. Further, they may be used together with the above-mentioned acidic components. The pH of the sol varies according to amounts of alkali metal, ammonium, amine, oxycarboxylic acid, etc. in the sol. If the pH of the sol is lower than 1, the sol becomes unstable, and if the pH is above 9, colloidal particles of tungsten oxide, stannic oxide and silicon dioxide complex are easily dissolved in a solution. If the total concentration of WO3, SnO2 and SiO2 is 40% by weight or higher, the sol also becomes less stable. If the concentration is too low, the sol is not practical, and thus the preferable concentration for industrial products is 5-30% by weight.
If the ultrafiltration method is used as a concentrating method, polyanions, quite fine particles and the like co-present in the sol are passed through an ultrafiltration membrane together with water, and thus polyanions, quite fine particles and the like which cause instability of the sol can be removed from the sol.
The step (d) is a step to mix 100 parts by weight, calculated as the total of ZrO2 and SnO2 contained therein, of the aqueous stannic oxidexe2x80x94zirconium oxide complex sol obtained in the step (b) and 2-100 parts by weight, calculated as the total of WO3, SnO2 and SiO2 contained therein, of the aqueous tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol having a particle diameter of 2-7 nm, a WO3/SnO2 weight ratio of 0.1-100 and a SiO2/SnO2 weight ratio of 0.1-100 obtained in the step (c) at 0-100xc2x0 C.
In the step (d), colloidal particles of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol are bonded on a surface of colloidal particles of the stannic oxidexe2x80x94zirconium oxide complex sol and the resultant surface is coated with the above-mentioned colloidal particles of tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex, to generate a modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles which have the colloidal particles as nuclei and which have surface properties of tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex. Further, the sol can be obtained in which the modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles are stably dispersed in the solvent of the solution.
The sol of stannic oxidexe2x80x94zirconium oxide complex colloidal particles modified with tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles is obtained by the step (d) of mixing 100 parts by weight, calculated as metal oxides thereof (ZrO2+SnO2), of the stannic oxidexe2x80x94zirconium oxide complex sol with 2-100 parts by weight, calculated as total of WO3, SnO2 and SiO2, of the above-mentioned tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex sol preferably with stirring strongly, and then the step (e) of removing anions in the sol from the mixed sol.
The modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles in the sol obtained by the mixing step (d) can be observed by means of an electron microscope, and they have a particle diameter of about 4.5-60 nm. Although the sol obtained by the above-mentioned mixing has pH of about 1-9, it contains a number of anions such as Clxe2x88x92, NO3xe2x88x92 and CH3COOxe2x88x92 derived from the oxyzirconium salt used for modification and thus micro-aggregation occurs in colloidal particles to lower transparency of the sol.
A stable sol of modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles having good transparency can be obtained at pH of 3-11 by the step (e) of removing anions in the sol obtained by the above-mentioned mixing step.
Anion removal in the step (e) can be attained by treating the sol obtained by the above-mentioned mixing with a hydroxide group type anion exchange resin at a temperature of 100xc2x0 C. or lower, preferably from room temperature to about 60xc2x0 C. As hydroxide type anion exchange resins, there may be used commercially available ones, but strongly basic type ones such as Ameliorate IRA-410 are preferable.
It is particularly preferable that the treatment by means of the hydroxide type anion exchange resin is carried out at a metal oxide concentration of 1-10% by weight in the sol obtained by the mixing step (d).
If the concentration of the modified stannic oxidexe2x80x94zirconium oxide complex obtained in the steps (a)-(e) is desired to be higher, it can be concentrated up to about 50% by weight by general methods such as an evaporation method and an ultrafiltration method. And, if pH of the sol is controlled, it can be carried out by adding the above-mentioned hydroxides such as above-mentioned hydroxides of alkali metals and ammonium, the above-mentioned amines and oxycarboxylic acids to the sol after concentrating. Particularly, the sol in which the total concentration of the above-mentioned metal oxides (ZrO2+SnO2) and (WO3+SnO2+SiO2) is 10-40% by weight is preferably in practice.
Colloidal particles in the modified stannic oxidexe2x80x94zirconium oxide complex sol obtained in the step (e) can be coated on their surfaces partly or totally with silane compounds such as ethyl silicate, methyltrimethoxysilane and xcex3-glycidoxypropyl-trimethoxysilane or hydrolysates thereof.
When the modified stannic oxidexe2x80x94zirconium oxide complex sol obtained by the above-mentioned mixing step is an aqueous sol, an organosol can be obtained by substituting the water medium of the aqueous sol with a hydrophilic organic solvent. The substitution can be carried out by general methods such as a distillation method and an ultrafiltration method. As examples of the hydrophilic organic solvents, there may be mentioned lower alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol; straight-chained amides such as dimethylformamide and N,Nxe2x80x2-dimethyl-acetamide; cyclic amides such as N-methyl-2-pyrrolidone; and glycols such as ethyl cellosolve and ethylene glycol.
The above-mentioned substitution of water with the hydrophilic organic solvent can be carried out easily by general methods such as a distilling substitution method and an ultrafiltration method.
Colloidal particles of the modified stannic oxidexe2x80x94zirconium oxide complex sol which are coated on their surfaces with colloidal particles of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex are negatively charged in the sol. The above-mentioned colloidal particles of the stannic oxidexe2x80x94zirconium oxide complex are positively charged, and colloidal particles of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex are negatively charged. Therefore, it can be considered as follows; colloidal particles of the negatively charged tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex are attracted electrically around colloidal particles of positively charged stannic oxidexe2x80x94zirconium oxide complex by the mixing step (d), and colloidal particles of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex are bonded on the surfaces of the positively charged colloidal particles by chemical bonding to coat the positively charged particle surfaces as nuclei with the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex and to form modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles.
However, when the stannic oxidexe2x80x94zirconium oxide complex colloidal particles having a particle diameter of 4-50 nm as the nuclear sol are mixed with the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles as the coating sol, the stable sol cannot be obtained if the total amount of metal oxides of the coating sol (WO3+SnO2+SiO2) is less than 2 parts by weight per 100 parts by weight of metal oxides of the nuclear sol (ZrO2+SnO2). It is considered that if the amount of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles is insufficient, the coating surfaces of the stannic oxidexe2x80x94zirconium oxide complex colloidal particles as the nuclei become insufficient, thus aggregation of formed colloidal particles occurs easily to make the formed sol unstable. Therefore, the amount of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles to be mixed may be less than that coats total surfaces of the stannic oxidexe2x80x94zirconium oxide complex colloidal particles but more than the minimum amount required to form the stable sol of the modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles. When the amount above that used for surface coating of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles is used, the obtained sol is formed as only just a stable mixed sol of the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particle sol with the modified stannic oxidexe2x80x94zirconium oxide complex colloidal particle sol.
Preferably, the amount of the tungsten oxidexe2x80x94stannic oxide silicon dioxide complex colloidal particles to be used may be less than 100 parts by weight of the (WO3+SnO2+SiO2) per 100 parts by weight of total metal oxides of the nuclear sol (ZrO2+SnO2) in order to modify stannic oxidexe2x80x94zirconium oxide complex colloidal particles by surface coating.
The preferable aqueous complex sol of the modified stannic oxidexe2x80x94zirconium oxide according to the present invention has pH of 3-11, and such a sol is liable to become unstable if pH is lower than 3. Further, if pH is above 11, the tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex by which the modified stannic oxidexe2x80x94zirconium oxide complex colloidal particles are coated tends to dissolve easily in the solution. Further, if the total concentration of the above-mentioned metal oxides (ZrO2+SnO2) and (WO3+SnO2+SiO2) is above 50% by weight, such a sol tends to become unstable. The preferable concentration for industrial materials is about 10-40% by weight.
Since tungsten oxidexe2x80x94stannic oxidexe2x80x94silicon dioxide complex colloidal particles tend to be hydrolyzed at a high temperature, the temperature of 100xc2x0 C. or lower is preferable during mixing in the step (d), anion exchange in the step (e) and concentrating, pH adjustment and solvent substitution after the step (e).